1. Field of the Invention
This invention relates to a tool comprised of a silicon nitride sintered body having a good wear resistance as well as excellent toughness, which is particularly useful in the field of cutting tools or wear resistance tools.
2. Description of the Prior Art
Up to the present time, in the field of art relating to tool parts comprised of sintered aluminum oxides, it has been proposed to increase the transverse rupture strength and improve the wear resistance characteristics of such tools by rendering the crystal grain size of the sintered body as small as possible and by simultaneously reducing the grain size distribution. Specifically, this proposal has been carried out by adding a sintering aid capable of lowering the sintering temperature, e.g. NiO, ZrO.sub.2, or Y.sub.2 O.sub.3, or by adding a sintering aid capable of suppressing the grain growth itself, e.g. TiC.
Increasing the transverse rupture strength and improving the wear resistance by rendering the crystal grain size of the sintered body as small as possible and by reducing the grain size distribution is also an effective means for improving the transverse rupture strength as well as the wear resistance of sintered silicon nitrides. Thus, some techniques have been developed to accomplish this object, for example, synthesis of fine grain high .alpha.-silicon nitride powder for the purpose of forming uniform nuclei of .beta.-silicon nitride in the transformation of .alpha..fwdarw..beta., optimization of the sintering curves, uniform mixing of a sintering aid such as Y.sub.2 O.sub.3, etc.
As a result of these efforts, silicon nitride sintered bodies have been employed as a tool material in the fields of cutting and machining cast iron, in which aluminum oxide sintered bodies are hardly used, for example, in the field of rough cutting cast iron.
As described above, the method comprising rendering the crystal grain size of the sintered body as small as possible and reducing the grain size distribution, thereby improving the transverse rupture strength as well as the wear resistance, is effective and has increasingly been used in the field of rough cutting cast iron, in which aluminum oxide sintered bodies are difficult to use. However, it is found by the inventors that when a silicon nitride sintered body is actually subjected to cutting of cast iron, wear of the silicon nitride sintered body proceeds rapidly by wear of the crystal grains through abrasion due to the presence of a very hardened layer (chilled layer) on the surface of the cast iron or due to the so-called "sand-containing" phenomenon. That is, it is apparent that there is a limitation for improving the transverse rupture strength and the wear resistance of a sintered silicon nitride tool by the method comprising rendering the crystal grain size of the sintered body as small as possible and reducing the grain size distribution.